It seems as though in military aviation, most governments want dedicated aircraft that are as efficient as possible for their mission. Along with that desire comes program delays, cost over runs, high unit cost, and alot of other "red tape" for every single aircraft progam it seems. Especially in the US this has been a large issue (tanker issue, VH-71, F-35, etc)

Way back when, it seems that militaries got ALOT more from a lot less. Look at the many variations of the C-130 and the C-135.

At what point would a single aircraft type filling multiple roles be more cost effective than the efficiencies gained from having a dedicated fleet?

Lets think about the commercial 767.. Once it is adapted/created for military use, there could be one freighter variant, one tanker variant, an AWACS (if needed) variant, etc

All coming off the same line, with small changes based on type seems to be an efficient way to gain many aircraft for an airforce, Instead of having an efficient aircraft with multiple packages, we have the Tankers, the Cargo aircraft, the old aircraft, mixed fleets, etc

The goal, of course, is low numbers that can be made to fit a multitude of jobs.

In theory, if you design an adaptable airframe from the start, you can save costs by reducing the number of different airframes and associated parts and training costs along the way. In practice it rarely seems to work out as well as the bean counters calculate.

Creating an adaptable airframe in itself carries an often unthought about series of problems and costs. Single role aircraft can, in principle, be designed and built relatively more efficiently than a similar number of multi-role designs. There are good reasons for this: limited goals allow for streamlining the design process and minimise the number of compromises made. Less complexity (a really good multirole aircraft is a particularly tricky task) often leads to less meetings, less redesigns, less calculations, less tests, less problems. That can equal less costs.

It's an interesting one, because if the multirole design pans out as calculated, the theoretical savings are very good compared to having two or more seperate designs. In the real world, things seem less clear. The complexities of wanting more for less seem to be increasing the delays, problems and costs. A number of multirole designs look like they may turn out to be false economies when looked back on with the benefit of hindsight, but at the moment who knows - I don't mean to judge here. I'm a historian in my spare time, so I'm looking at today with an eye to what has gone before, but the landscape is not the same, though it looks similar.

Many of the best multirole designs seem to have their origins in single role aircraft, rather than be designed as multirole from the outset. It's usually never planned that way and you can never be sure what other roles any design may be good at when you first introduce it, so it's a bit of a lottery. I don't think anyone expected the F-15 to be such a good mud-mover when it was designed as an air superiority fighter, but the strengths it gained from that role turned out to be equally good for the ground attack role - this time. There have been plenty of air superiority fighters in the past that could never make that transistion. McDD did a good job. We shouldn't forrget that many of those aircraft which we now remember as great successes also had their own budget over runs and delays.

Might be fun to come up with a list of single role aircraft that excelled in areas they were never designed for. Likewise, might be fun to list some multi-rolers and see how history judged their performance too.

I think the US did really well with the Hi-Lo combo they got with the F-15 and F-16. They are trying that again with the F-22 and F-35, but I think too much multi-role from the outset has been designed into the F-35. Maybe it will just be the C model that is a problem and the other two will shine, we'll have to see.

Modern designs are increasingly complex and expensive. Economics is more important than ever and this pushes the benefits of a good multirole aircraft up the agenda. The problem is, for the gamble to pay off, that multirole aircraft must deliver all it's performance promises, all it's cost promises and all it's savings promises.

Some aircraft designs work very well adapting to future missions from their original mission. The KC/C-135, military B-707, and C-130 are to great examples, as you already said. Other aircraft designs work best in original mission designs, the B-52, F-22, and F-15 (not including the F-15E). Other designs just lend themselves to just a few additional missions than what the were designed for, and do that very well, like the F/RF/FB/EF-111 did (the FB-111 was a strategic bomber, while the F-111 was a tactical bomber). Some designs are very good at different missions, but not great at any one mission, the F/RF-4 is an example of this, good at fighting, bombing, and recee, but great at none.

Then there are the airplanes that did not do as well as expected in their primary mission, but excelled at other missions. The A-3/B-66 is an example of these, just okay at bombing, but very valuable at jamming, refueling, recee, and other missions.

The jury has not even been seated to decide how good, or bad the F-35 will be.

Quoting KC135TopBoom (Reply 3):Then there are the airplanes that did not do as well as expected in their primary mission, but excelled at other missions. The A-3/B-66 is an example of these, just okay at bombing, but very valuable at jamming, refueling, recee, and other missions.

Another interesting category relevant to this topic and an excellent example it is too. Another example I can think of here is the Supermarine Swift which proved constantly troublesome and flawed as the fighter it was designed to be, but turned out to be a very useful low-level recce machine.

The past of aircraft design is littered with designs that were excellent in their intended role, dreadful in their intended role, good at many things, good at things they were never intended to do and so on. Some designs start out failing but eventually get fixed and 'come good'. Some designs meet their brief perfectly only to find out the role they were dsigned for is now obsolete.

Advances in knowledge, computing and materials are matched by increasing demands on designs. It's a fascinating 'war' for the engineers - the better they get the more is expected of them. It's very exciting, but it does mean that constantly pushing the boundaries leads to the risk of failues (be they in role, costs, time etc.) remaining much as it always has done. Its the price you might pay for staying cutting edge and leading the field (remember the DH Comet?).

Quoting KC135TopBoom (Reply 3):The jury has not even been seated to decide how good, or bad the F-35 will be.

No, you are right. It will be along time before we can look back and see how it did. I hope the C makes it. It's a bold machine, a tough ask and it's having a tricky birth. I don't like to see people taking the heat for trying to push the boundaries of the possible, if it does not work out.

I'll admit to being a little concerned that in recent years, manufacturers (civil and military), have been a little too ambitious with their designs - at least compared to the timescales and costs that they have budgeted for. Every time a new aircraft (or ship, sub, or weapons system) is announced, the first thing I'm thinking is how late and over-budget will it be. It's a pity that no-one is allowed to be honest about these things and be able to say "It's an ambitious design, we can do it, but we'll have troubles along the way which we can solve. However, as a result we don't know exactly when it will be ready or what the final cost will be, but we can do it". I understand why that can't happen (accountabilty - no blank cheque books), but a little more genuine honesty and flexibility on both sides might help things through.